The present disclosure pertains to an apparatus and a method for controlling modes in a semi-guiding amplifier medium.
Fiber lasers can be used in many applications and are increasingly sought after in certain applications as substitutes to solid state lasers. Fiber lasers are more compact, reliable have high efficiency, and can provide robust single-mode output. In addition, fiber lasers can be used in an all-fiber architecture without free-space optics and hence may not require a rigid optical bench. Over the past decade, output powers of fiber lasers have been increased several orders of magnitude, from the watt-level to multi-kW powers, making fiber lasers competitive with solid state based lasers.
One of the challenges inherent in scaling the fiber laser output power is selecting lower order modes from a plurality of modes in the fiber laser and substantially reducing or eliminating higher order modes to ensure stable high beam-quality output. If higher modes are allowed to compete for the available pump power of the laser, the higher modes in the laser can be amplified and significantly degrade the output beam quality and may reduce the lasing efficiency.
In conventional fiber laser systems, mode control is generally accomplished by providing a core of fiber laser that causes higher order modes to radiate out into the cladding of the fiber laser. For example, for large mode area (LMA) fiber lasers, a mode-dependent loss may be created by forming the fiber laser into a coil with a predetermined bend radius. Coiling imposes radiation losses that are highly dependent on mode order. The loss rate increases rapidly with increasing mode order. Hence, using a proper coiling radius the higher-order modes can be stripped out leaving only the lower-order modes. Alternatively, a secondary core can be wound as a helix around the primary core of the fiber laser at a predetermined pitch and distance from the primary core of the fiber laser. In this case, the secondary core wound as a helix around the primary core can draw out the higher-order modes. However, these types of mode control may be impractical for fiber lasers having a relatively large diameter core (i.e., a diameter exceeding 50 μm) since mode discrimination becomes inadequate between the increased number of competing modes and the diminishing separation in loss rates between neighboring modes to reliably select the lowest-order mode while operating at a low transmission loss.
Another conventional approach in controlling modes in a fiber laser is the use of higher order mode (HOM) fiber lasers. HOM fibers have a large numerical aperture NA. In HOM fibers, the circular core (e.g., with a diameter approaching and even exceeding 100 μm) is multimode and a signal beam can propagate through the fiber as a single but higher-order linearly polarized (LP1m) mode. The higher mode is preferred in this case because of its large effective area and its strong immunity to fiber bends. Special couplers that convert between the lower-order modes and the preferred higher-order mode are employed at each end of the HOM fiber. However, in these types of fiber lasers, power scalability is limited by the need of a narrow core LMA-based output coupler to convert the higher order mode to a conventional high quality LP01 output beam, and limited by optical coupling between different LP modes due thermal gradient arising inside the core of the fiber laser.
Another conventional approach in controlling modes in a fiber laser is by employing gain guiding. Gain guiding occurs when the active core is surrounded by a cladding having either an equal or slightly higher refractive index to make the core anti-guiding. In this case, all the modes for the core are lossy due to the anti-guiding effect that pulls the modes into the cladding. However, if the core is optically pumped to provide gain, the lower-order mode gravitates to, or tends to concentrate within, the active region and can be optically trapped by the gain region itself. However, gain guiding is extremely sensitive to fiber bends and can be highly susceptible to small refractive-index variations. As a result, gain-guided fiber lasers cannot be coiled. Gain guided fiber lasers are also highly sensitive to very small index inhomogeneities, and the fact that the mode size depends on the gain adds to the complexity of this approach.
Another conventional approach in controlling modes in fiber lasers is by employing gain filtering. Gain filtering has been utilized in index-guided round core fiber lasers. In this case, a compact gain region is provided and is localized to a fraction (approximately 50%) of the core diameter which allows the lowest-order mode to have higher gain due to the greater overlap of the lowest-order mode with the compact gain region. However, gain saturation limits the effectiveness of gain filtering to about 5 to 10 modes for a typical gain of about 100.
Yet another conventional approach in controlling modes in fiber lasers is to use a leakage channel fiber (LCF). FIG. 12 depicts schematically a cross-section of a leakage channel fiber. Fibers typically guide the signal in the core by surrounding the core with a continuous cladding material having a refractive index slightly lower than that of the core. In the LCF architecture shown in FIG. 12, the cladding is replaced with an annular array of holes (they could be air holes, or holes filled with a low-index glass) of diameter d. Each gap between the holes represents a “channel” through which the power in the core can leak out. The parameters of the LCF (specifically d and Λ) can be optimized to yield a much greater loss for higher-order modes than for the fundamental (lowest-order) mode. This mode-dependent loss can be exploited to allow the fundamental mode to dominate over all others. However, the LCF architecture suffers from the fact that the channels between the holes that allow mode discrimination also lead to significant losses of the fundamental mode (lowest-order mode) when the fiber is formed into a coil with a relatively small diameter. For example, in the LCF architecture, for 2p=100 μm (appropriate for a high-power fiber), the bend radius required to maintain an acceptable fundamental-mode loss less than 0.1 dB/m is 1.25 m (i.e., a full diameter of 2.5 m), thus making the LCF architecture not amenable to coiling in a small diameter package. A further drawback of the LCF architecture is that, as the core diameter is increased to accommodate higher powers, the path length from the core to the cooled fiber surface becomes longer, making thermal management a challenge. The thermal load manifests itself as a strong radial temperature gradient, which can result in thermally induced lensing and birefringence, and, in extreme cases, fracture.
None of the above conventional approaches addresses controlling modes in a semi-guiding amplifying medium or in a semi-guiding high aspect ratio core fiber laser.